Renal tubular transport of proteins is felt to occur by endocytosos. In this process, the protein is recognized, bound to the brush border membrane, and internalized in a vesicle. The factors which are important for recognition, binding, and internalization of the various proteins handled by the kidney are incompletely understood. In addition, quantitative evaluation of the renal capacity to absorb filtered proteins is available for relatively few proteins. The renal tubular capacity and affinity for absorption of Bence Jones proteins, in particular, is poorly defined. In this study, renal tubular handling of a group of four homologous kappa light chains (Bence Jones proteins) will be examined with the technique of in vivo tubular microperfusion in rat superficial nephrons. Each of these proteins has a different net electric charge with isoelectric points ranging from 4.3 to 7.6. These proteins, synthesized by immunocytomas implanted in rats, are collected in the urine, isolated, and radiolabelled with 125I. Using micropuncture techniques, 125I-light chain is perfused into one end of a tubule segment. At the other end, the perfusate is collected and the amount of 125I-light chain absorbed over the length of the segment is determined. The affinity and capacity of the tubule to absorb each of the differently charged light chains will be evaluated under conditions of varying light chain loads. The effect of co-infusion of the other light chains, and variation of the infusion amino acid composition on absorption of these proteins will also be examined. The magnitude of absorption of the different light chains will be analyzed to see if: 1) they are similar or different, b) they suggest the absorption is receptor mediated, c) electric charge constraints seem to be important. Morphologic studies will be done to see if the differently charged proteins are bound to different sites on the tubular luminal membrane, if they are internalized in different ways, and if exposure of the tuvules to these light chains induces cell toxicity. Results of this study will enable a better understanding of the mechanisms by which renal tubular protein transport takes place. Moreover, knowledge of the factors which determine the capacity and affinity of the kidney to absorb different Bence Jones proteins may be of clinical import in evaluating the patient with multiple myeloma.